This invention relates to mass rate of flow meters of the angular momentum type having a swirl generator for imparting swirl to the measured fluid stream and a torque balance reaction generator for removing the imparted swirl. More particularly, this invention relates to such a meter having an improved readout system for indicating the mass rate of flow.
This invention is particularly adapted for use in a mass rate of flow meter which utilizes a spring-restrained turbine as the torque balance reaction generator. One such mass rate of flow meter is depicted in U.S. Pat. No. 4,056,976 issued Nov. 8, 1977 and titled Mass Rate of Flow Meter, which patent is assigned to the same assignee as the present invention. This meter includes a housing that defines a fluid passage that extends along a longitudinal axis through the housing and that has an input port and an output port located on the axis. A swirl generator is located adjacent the input port to impart a substantially constant angular velocity to an entering fluid stream. As the fluid leaves the swirl generator, it passes through an axially displaced, unrestrained rotor that rotates about the axis. The angular velocity of the rotor accurately represents the angular velocity of the fluid stream as it leaves the rotor and passes through an axially spaced, spring-restrained turbine. The angular momentum of the fluid stream angularly displaces the turbine about the axis and against the bias of its restraining spring. Under steady state conditions, this deflection of the turbine is proportional to the mass rate of flow.
In a spring-restrained flowmeter, the rotor carries two circumferentially and longitudinally displaced bar magnets. The first magnet is disposed on the input end of the rotor and is circumferentially poled. A first sensing coil assembly in a transverse plane through the first magnet is radially spaced from the magnet and isolated from the fluid flow. Each time the first magnet passes the first sensing coil, it induces a "start" voltage pulse in the coil that indicates the passage of a predetermined point on the rotor past a predetermined point on the housing.
The second magnet is at the exit end of the rotor and diametrically opposed to the first magnet. An axially disposed, longitudinally extending bar of a highly permeable material, such as soft iron, mounts on the periphery of the turbine. The axial spacing between the rotor and the turbine interposes an axial air gap between the bar and the second magnet when they align. A second sensing coil assembly, that is isolated from the fuel flow, is coaxial with and longitudinally coextensive with the second magnet and the bar. Each time the second magnet passes the bar, the flux that the bar couples to the second sensing coil assembly changes and induces a "stop" voltage pulse in the second sensing coil. As described in the foregoing U.S. Pat. No. 4,056,976, timing circuits convert the start and stop pulses from the first and second sensing coil assemblies into an indication of the mass rate of flow through the meter.
There are, therefore, two separate, magnetic circuits in this mass rate of flow meter. The first, defined by the first magnet and the first sensing coil assembly, includes a radial air gap. The second, including the second magnet and the highly permeable bar, includes an axial air gap. During use of the mass rate of flow meter, there are several influences that tend to change the lengths of these air gaps, although to different degrees. Two such influences are vibrational forces and thermal expansion.
A typical application for such a flowmeter is in an aircraft for measuring the mass rate of flow of fuel to an engine. The engine and air frame are sources of mechanical vibrations. Also, the fuel that passes through the flow meter is subject to pressure pulsations that introduce other mechanical vibrations. These vibrations have radial and axial components that are coupled to the flowmeter and produce corresponding radial and axial forces that act on all the elements in the flowmeter. However, with the construction of the flowmeter, including the methods for mounting the rotor and turbine on a shaft for rotation, the individual elements are more stable in a radial direction than they are in an axial direction and, therefore, are less likely to vibrate in a radial direction.
In addition to vibrationally induced forces, the elements also are subjected to the forces of thermal expansion. When an aircraft is flown, the temperature of the fuel varies widely. Any elements that contact the fuel, including the rotor and the turbine of the flowmeter, are therefore subject to these temperature variations and, therefore, to thermal expansion. Both the rotor and turbine are characterized by material dimensions in the radial direction (i.e., radial thickness) that are significantly less than those in the axial direction (i.e., axial length). Thus, the radial effects of thermal expansion forces are less than the axial effects.
During operation of the flowmeter, the radial forces that are produced by vibrations and thermal expansion do not vary the length of a radial air gap sufficiently to affect the ability of the first sensing coil assembly to generate detectable pulses on a reliable basis. However, the axial forces that are produced by vibrations and thermal expansion can, in some environments, adversely affect the signal-to-noise ratio of the signal that is produced in the second sensing coil. Moreover, the problems can be increased depending upon the axial motion that is permitted by manufacturing tolerances. It is possible, under some circumstances, for the signal level from the second sensing coil assembly to reach undetectable levels and, under other circumstances, for the noise to reach a level at which it is detected as a signal. Either situation leads to erroneous flowmeter readings.